1. Field of the Invention
The present invention relates to cationic resins and to their use in electrodeposition, and particularly to cationic resins containing capped isocyanate groups and to their use in electrodeposition.
2. Brief Description of the Prior Art
The application of a coating by electrodeposition involves depositing a film-forming composition to an electrically conductive substrate under the influence of an applied electrical potential. Electrodeposition has gained prominence in the coatings industry because in comparison with non-electrophoretic coating methods, electrodeposition provides higher paint utilization, outstanding corrosion resistance, and low environmental contamination. Early attempts at commercial electrodeposition processes used anionic electrodeposition, where the workpiece being coated served as the anode. However, in 1972, cationic electrodeposition was introduced commercially. Since that time, cationic electrodeposition has become increasingly popular and today is the most prevalent method of electrodeposition. Throughout the world, more than 80 percent of all motor vehicles manufactured are given a primer coating by cationic electrodeposition.
To maximize corrosion resistance over steel substrates, cationic electrodeposition compositions are conventionally formulated with lead as either a pigment or a soluble lead salt. In addition, the steel substrate is usually pretreated before electrodeposition with a phosphate conversion coating and rinsed with a chromic acid solution (chrome rinse) after pretreatment. However, lead and chromium are believed to be very environmentally undesirable. The lead may be present in the effluent from electrodeposition processes and chromium may be present in the effluent from pretreatment processes, and these metals need to be removed and disposed of safely, which requires expensive waste treatment processes.
U.S. Pat. No. 5,074,979 discloses a cationic resin suitable for use in an electrodepositable composition which is free of lead and has excellent corrosion resistance. However, this composition does not provide optimum chip resistance. Moreover, such a system is unstable upon storage, generating carbon dioxide gas.
It would be desirable to provide an electrodepositable composition which does not require the use of lead and overcomes the limitations of the prior art.